It is well known that many metals, such as copper, may be continuously cast in a rotating casting wheel to obtain a cast bar which is then immediately hot-formed in a substantially as cast condition by passing the cast bar exiting the casting wheel to and through the roll stands of a continuous rolling mill while the cast bar is still at a hot-forming temperature. It is also well known that the as cast structure of the metal bar is such that cracking of the cast bar during hot-forming may be a problem to be overcome if the cast bar is to be directly hot-formed into a small product, such as coiled rod, which requires that the desired cross-sectional area of the cast bar be substantially reduced by a plurality of deformations along different axes to provide the cross-sectional area of the product.
While this problem could be avoided by casting a cast bar having a small cross-sectional area which need not be substantially reduced to provide the cross-sectional area of the product, this solution to the cracking problem described above is not practical since high casting rates, in terms of volume of product, can be readily achieved only with cast bars having large cross-sectional areas which are reduced to the small cross-sectional areas of products, such as 3/8 inch diameter rod for drawing into wire, by a plurality of deformations along different axes. Alternately the problem may be avoided by providing a large number of roll stands which each reduce the area of the cast bar by only a small amount. However the great expense of continuous rolling mills requires that the number of roll stands be minimized. Thus, the problem of a cast metal cracking during hot-forming which is described above must be solved within the context of cast bars having large cross-sectional areas which are hot-formed into products having small cross-sectional area and the use of heavy reductions which may be substantial enough to cause cracking of the cast bar if the problem is not overcome in some manner.
This problem has been overcome in the prior art for relatively pure electrolytically refined copper having impurity levels such as 3 to 10 ppm lead, 1 ppm bismuth and 1 ppm antimony. For example, U.S. Pat. No. 3,317,994, and U.S. Pat. No. 3,672,430 disclose that this problem can be overcome by conditioning such relatively pure copper exiting from a casting wheel as a cast bar by initial reductions of the cross-sectional area in the initial roll stands sufficient to substantially completely destroy the as-cast structure of the cast bar. The additional reductions along different axes of deformation which would cause cracking of the cast bar but for the destruction of the as-cast structure of the cast bar may then safely be performed. This conditioning of the cast bar not only prevents cracking of the cast bar during hot-forming but also has the advantage of accomplishing a large reduction in the cross sectional area of the cast bar while its hot-forming temperature is such as to minimize the power required for the reduction. However, the prior art has not provided a solution to the cracking problem described above for metals, such as recycled copper, containing a relatively high degree of impurities, because the large amount of impurities collect in the grain boundaries of the as-cast structure and cause the cast bar to crack when an attempt is made to substantially destroy the as cast structure with a large initial reduction of the cross sectional area (e.g. 36%) which is effective with lower impurity metals. Moreover, the greater the percentage of impurities in the cast bar, the more likely it is that cracks will occur during hot forming.
Thus, although there is no need as a practical matter to use electrolytically refined copper, except for specialized uses such as magnet wire, it has been necessary to use such highly refined copper for all wire rod in order to be able to obtain the many advantages of tandem continuous casting and hot forming apparatus. As a result, a substantial refining cost is added to the price of many final copper products even though high purity is not required to meet conductivity or other specifications. For example, fire-refined copper wire having a high degree of impurities can meet the IACS conductivity standard for household electrical wiring and can be produced more economically if the rod to be drawn into such wire can be produced using continuous casting and hot forming apparatus.
U.S. Pat. No. 3,317,994, U.S. Pat. No. 4,129,170 and U.S. Pat. No. 3,349,471 illustrate the state of the art in continuously casting molten metal into bar and immediately rolling it into rod. In such systems hot bar is passed through a series of reduction roll stands and emerges as a rod which is collected for further processing. Depending on the quality and type of metal being cast, substantial initial reduction is often advantageous in controlling grain structure, segregation, invese segregation, uniformity of inclusion dispersal, and productivity. Until the present invention, initial reduction has been limited because when using greater degrees of initial reduction, metal bar of a lower purity level will often crack. Thus this cracking problem occurs most often in lower grades of metal, even at relatively low reductions, as well as in higher grades of metal at the higher reductions.
Less well known in the art is the effect of hydrostatic pressure on these same metals. However, it has been an observed fact for quite some time that highly brittle materials such as sandstone and marble show appreciable ductibility when subjected to very high hydrostatic pressure. The level of ductility evidently increases with increasing pressure. An article by P. W. Bridgman appearing in Reviews Of Modern Physics, Volume 17, Number 1, January, 1945 entitled "Effects of High Hydrostatic Pressure on the Plastic Properties of Metals" discusses effects at various high pressures such as 34,000 p.s.i. to 387,000 p.s.i. The related subject of hydrostatic extrusion is addressed by U.S. Pat. No. 2,558,035 wherein material is cold drawn through a die while under influence of 400,000 p.s.i. hydrostatic pressure. Until the present invention, however, the problem of cast metal cracking during continous hot rolling has not been satisfactory dealt with nor have the effects of lower hydrostatic pressure been investigated in this regard.